The oscillatable unit can be differently embodied, depending on application: as an oscillatory fork having two fork tines arranged symmetrically on a membrane (FIG. 1a), as a single rod, in the case of which only one tine is arranged centrally on a membrane (FIG. 1b) or as a membrane alone (FIG. 1c). Vibration sensors with oscillatory forks are applied in liquids, gases and solids and are manufactured and sold by the applicant under the mark LIQUIPHANT. Known under the mark SOLIPHANT are vibration sensors with a single rod. These are designated mainly for use in solids. Known from German Patent DE 10 2005 044 725 A1, moreover, is an embodiment of a membrane oscillator, which is suitable for use in the most varied of media.
Vibronic sensors oscillate at a defined resonant frequency so that they execute a harmonic oscillation. The resonant frequency is determined by the construction of the sensor and the materials used. Oscillations can be characterized by frequency and damping. If the oscillatable unit oscillates in a liquid medium with a high density, the density of the medium, as a coupled, moved mass, has an influence on the oscillatable unit. As a result, the oscillation frequency in a liquid medium is lower than in a gaseous medium. A frequency change indicates, thus, for example, a transition from a gaseous state to a liquid state of a medium. Furthermore damping by the medium has an influence on the oscillations of a vibronic sensor. Bulk goods such as wheat or rice damp the oscillations of the oscillatable unit of a vibration sensor and bring about a drastic amplitude decrease at the transition air/bulk good.
Vibration sensors embodied as limit level measuring devices, thus, utilize the effect that both the oscillation frequency as well as also the oscillation amplitude depend on the respective degree of coverage of the oscillatory element: While the oscillatory element can in air execute its oscillations freely and without damping, it experiences frequency and amplitude changes, as soon as it becomes immersed partially or completely in the medium. Based on a predetermined frequency change (usually frequency is measured), it can be clear that a predetermined fill level of the medium in the container has been reached. The frequency change in non-damping media such as gases and low viscosity liquids depends on the density of the medium. The frequency change is sufficient to detect the medium and to evaluate its density. Fill level measuring devices are used, moreover, principally as overfill preventers or for the purpose of protection against a pump running empty.
As already indicated, the damping of the oscillation of the oscillatable unit is predominantly determined by the frictional forces of the solid particles or molecules of the respective medium. Therefore, in the case of constant degree of coverage, there is a functional relationship between oscillation amplitude and density of a bulk good (the friction in heavy bulk goods with a high bulk good density is higher than in the case of lighter bulk goods) or between the oscillation amplitude and viscosity, so that vibration sensors are suitable both for fill level measurements as well as also for density determination in bulk goods. Furthermore, vibronic sensors are applied for determining the viscosity of a liquid medium.
The oscillations of a vibration sensor are produced by an electromechanical transducer. The electromechanical transducer is usually a piezo drive having at least one piezoelectric element. The piezo drive excites in the vibration sensor harmonic oscillations of a resonant frequency and compensates for energy losses, which occur in the oscillatable unit. Piezo drives can achieve a high efficiency. Since the energy supply is relatively small, wide application in automation technology is possible. Further information can be found, for example, in German Patent DE 10 2008 050 266 A1. Often so-called stack drives are applied as piezo drives. In the case of stack drives, a number of disk-shaped piezoelectric elements are arranged stacked on top of one another. Moreover, bimorph drives are used for oscillation production and oscillation detection. In principle, a bimorph drive is composed of a disk shaped piezoelectric element connected with the membrane by a force transmitting connection, wherein the piezoelectric element has opposite polarization in at least two areas. European patents EP 0 985 916 A1 and EP 1 281 051 B1 describe different embodiments of bimorph drives.
In the case of fill level determination, the evaluation unit monitors the oscillation frequency and/or the oscillation amplitude of the oscillatory element and signals the states ‘sensor covered’, respectively ‘sensor uncovered’, as soon as the measurement signals subceed or exceed a predetermined reference value. A corresponding report to operating personnel can occur by optical and/or acoustical means. Alternatively or supplementally, a switching event is triggered; thus, for instance, a supply or drain valve on the container is opened or closed.
Piezo technology based on LZT (lead zirconate titanate) piezoceramic materials is best suitable for use at temperatures up to 300° C. There are piezoceramic materials, which keep their piezoelectric properties at temperatures above 300° C. These have, however, the disadvantage that they are markedly less effective than the LZT-based materials. Such high temperature materials are little suitable for use in vibration sensors.
The main impediment for application of piezoceramic materials in vibration sensors at temperatures above 300° C. is the great difference in the thermal coefficients of expansion of metals and ceramic materials. The piezoceramic elements act as force providers in vibration drives: Therefore, the piezoelectric, respectively piezoceramic, elements must be connected by a force transmitting connection with the membrane, which is usually manufactured of stainless steel. Due to the different thermal coefficients of expansion, mechanical stresses in the piezoceramic elements get so high with rising temperature that the piezoceramic elements eventually fracture—the result is a total failure of the vibration sensor.
In order to avoid these problems, Published International Applications WO 2007/113011 and WO 2007/114950 A1 describe vibration sensors, which use a special electromagnetic drive. Essential components of the electromagnetic drive are a coil and a permanent magnet. If the electromagnetic drive is supplied with an alternating voltage signal, then an alternating magnetic field is produced. As a result of the alternating magnetic field, a periodic force acts on the oscillatable unit of the vibration sensor and excites it to execute oscillations. In the case of this known sensor, the changing of electrical energy into mechanical energy occurs via a magnetic field. In the case of an electromagnetic drive, the differences of the thermal coefficients of expansion of the materials in the sensor are of lesser significance. Since in contrast to the piezoelectric drives a force transmitting connection between two completely different materials, such as e.g. the stainless steel membrane and the piezoceramic, does not need to be used, a vibration sensor with an electromagnetic drive is also applicable in a higher and broader temperature range, especially temperatures between −200° C. and 450-500° C.
A disadvantage in the case of the known vibration sensors with electromagnetic drive is that a permanent magnet interacting with a coil as force provider has a clearly lesser efficiency than a piezo drive. While the electromagnetic drive develops relatively high forces in the region of the membrane, nevertheless the deflection of the oscillatory fork as a result of the non-force transmitting connection between membrane and drive is small. As a result thereof, a vibration sensor with electromagnetic drive requires more energy in comparison to a vibration sensor with piezo drive. This makes its use in explosion-endangered regions problematic.
U.S. Pat. No. 3,256,738 discloses a magnetostrictive sensor for detecting the limit level of a medium in a container. The sensor housing is also in this case sealed on its underside with a membrane. A tubular component of a magnetostrictive material extends into the housing interior and is welded in one of its end regions with the central region of the membrane. Force transmission can occur through the weld. The second end region of the tubular component is free. Located in the outer region of the tubular magnetostrictive component as transmitting/receiving unit are two coils with an annular permanent magnet lying therebetween. In the case of the known solution, small tubes of magnetostrictive material are excited to execute longitudinal resonant oscillations. Longitudinally oscillating resonators have a high mechanical quality and react with an amplitude change, as soon as they come in contact with the medium to be monitored. The known sensor is excited to execute resonant oscillations by changing the tube length in a harmonic magnetic field.
A disadvantage of the known magnetostrictive sensor is that it is not mechanically decoupled from the container wall. Depending on connection, there is the danger that it will stop working. For exciting oscillations in applicant's vibration sensors, which are applied as safety switches, the known magnetostrictive drive is not suitable.